In the completion of oil and gas wells it is common practice to cement a casing or liner in a borehole and thereafter, perforate the casing or liner at one or more desired locations to provide flow paths into the casing or liner for the flow of oil and/or gas from the formation surrounding the casing for production of oil or gas. Typically a casing or liner in a completion operation contains a fluid such as drilling mud or other suitable fluid which provides sufficient hydrostatic pressure above the pressures encountered in the surrounding earth formations to prevent a well blowout. In perforating casing in earth formations where the oil or gas is under pressure in the formations, a number of variables are taken into consideration with respect to the productivity of oil or gas. For example, the projected depth of penetration into the earth formations, the number of perforations per foot of casing, the angular displacement of the various perforations around the axis of the casing and the diameter of the perforations are parameters affecting productivity. Additionally, the differential pressure, i.e., the difference in the pressure between the pressure in the earth formations penetrated by the perforations and the pressure in the interior of the casing at the time of perforating affects the productivity from the earth formations.
The differential pressure is a positive pressure when the pressure of the fluid column in the casing exceeds the pressure of the fluid in the reservoir or earth formations. In some instances the drilling mud pressure is from 500 to 1000 psi greater than the formation pressures. This positive pressure provides good well control and is considered by many operators to be the desired pressure mode where the perforations are made with large size perforating guns. However, while high penetration is achieved with larger perforating guns, the perforations in the formations are frequently plugged or clogged up by drilling mud which under higher pressure forms a filter cake on the fresh perforation opening. This can largely negate the advantages of the deeper penetration and larger hole size obtained by larger perforating guns.
Contrasted to a larger perforating gun and a positive pressure completion, a later developed method uses a through-tubing perforating gun which passes through tubing attached to a packer to a location below the packer. Through-tubing guns are smaller than the conventional casing guns and are typically fired in a negatively balanced well bore; that is to say, with the pressure in the casing and in the tubing being lower than the pressure in the surrounding earth formations. The through-tubing perforating gun is necessarily smaller and therefore does not produce the depth of penetration and hole size in an earth formation as compared to the conventional casing gun but does have higher shot density. Thus, while a through-tubing gun can achieve high effective shot density (shots per foot), reduced penetration and reduced hole size reduces the productivity which makes the technique unattractive for deep wells where high temperature, high pressure and hard formations exist. Also, with the throughtubing gun arrangement, well pressure control is a source of concern because, upon firing of the gun, the entire length of the tubing string is subjected to an elevated pressure and the well is controlled only by means of wireline pressure control equipment at the surface. Further, where negative differential pressure is employed to effect good perforation cleanup by reverse or back flow from the perforations to the casing, low permeability formations require a very high negative differential pressure to clean up the perforations, especially when gas is the produced fluid. Thus, when a high negative differential pressure is required for cleaning up the perforations there is a hazard of blowing the gun and its supporting cable up the casing which can result in the gun and cable becoming fixed or jammed in the well. This then requires an expensive fishing operation or, much worse, the well may have to be killed by high pressure control fluids which can possibly permanently damage the formations.
Another completion technique is when a tubing string carries a large, high performance gun below the packer and the gun fired after setting the packer and providing negative pressure in the tubing. While this system provides large gun performance and deep perforations and prevents blowing the gun and a cable up the hole, the tubing string is subjected to sudden large increases in pressure with the possibility of failure and loss of control of the well. Moreover, in the event a gun misfires, the entire tubing, packer and gun assembly must be withdrawn with subsequent significant increases in time and cost.
The method and apparatus of the present invention preferably employ a negative pressure technique for perforating and provide a means for safely maintaining well control with a full head of hydrostatic fluid or positive pressure above the perforated zone. In this system a way is provided to isolate the perforated zone at negative pressure from the hydrostatic fluid at positive pressure to induce back flow into the wellbore immediately upon perforating. The negative pressure and back flow from the perforations cleans the newly formed holes by expelling perforating gun debris and formation debris from the perforations rather than leaving such debris in the perforations and permitting invasion by drilling mud at a positive pressure.
The present invention results in higher productivity than with the through-tubing perforation guns because larger perforating guns can be run below a packer on a tubing and, when operated in conjunction with a PBR (polished bore receptacle) the gun can be even larger than when using a packer on a tubing. Further, the apparatus of the present invention minimizes the problem of debris plugging the perforations in that high negative differential pressures across the perforation can be used with positive differential pressure in the tubing string which eliminates adverse pressure effects such as creating sudden and excessive surface pressure on the wireline control or surface equipment and eliminates the risk of blowing the gun and its support cable up the casing or tubing. The present method enjoys the additional advantage of providing full well control at all times, i.e., if for any reason the packer on the tubing should fail at or after firing of the perforating gun, there is sufficient fluid above the packer at a positive pressure to kill the well should that be required. Thus, the apparatus and method of the present invention permits use of higher performance guns while maintaining well control throughout the perforating activity.